Batteries are now in use in a wide range of applications. This range of applications is expected to increase in the future as storage battery technology, particularly energy density, continues to improve. In recent years, implanted biomedical batteries have become important for powering so-called bionic devices such as cochlear implants, neuromuscular stimulators, cardiac pacemakers and defibrillators, and artificial organs. In addition, batteries have become an essential power source in a wide range of portable electronic devices including computers, personal information managers, radio telephones (“cellular telephones”), global positioning satellite (GPS) devices, and other devices that combine the functions of the foregoing. The safety of these devices is paramount as the explosion of a battery in any of these devices could cause injury and death.
Batteries may present certain safety problems under a variety of circumstances. These potential problems can become more acute, even life threatening, when they are implanted in a human being. These problems may include internal short-circuits, over-pressure leading to bulging enclosures, electrolyte leaks, explosion, over-heating and combustion. Such faults can result from both internal and external factors. They cannot be tolerated in any implanted battery, and could lead to serious safety problems in any application.
The present invention is fundamentally an emergency energy drain or “dump” system. That is, in the event of a serious fault such as an internal short circuit, the remaining energy in the battery is quickly discharged by diverting it to a discharge radiator or sink acting to safely dissipate or absorb the heat generated by the current so diverted. While it is commonly accepted in the art that rapidly draining a battery would cause it to dangerously increase its heating, tests have shown that this counter-intuitive approach is highly effective in preventing dangerous over-heating and/or rupture of batteries that experience an internal short circuit.
The present invention is particularly suited for human-implantable batteries, however may be applied to any electrochemical storage device, or even inertial energy storage devices such as flywheels.
Battery safety circuits and devices in general are widely used in both primary (disposable) and secondary (rechargeable) batteries and charging circuits. The circuits typically limit charging and discharging, or disconnect a battery in the event of over-heating or over-pressure in the battery. These devices are intended to prevent real-world failures, but are also designed to meet certain industry and regulatory test requirements such as nail penetration and mechanical crush tests.
U.S. Pat. No. 6,210,824 issued to Sullivan, et al., for example, discloses an end-cap incorporating a pressure sensitive switch intended to disconnect the battery from a circuit in the event pressure inside the battery casing becomes excessive.
Similarly, U.S. Pat. No. 5,766,793 issued to Kameishi et al. discloses a bi-metal plate that bends when heated due to overcharging or short circuiting, breaking the external circuit.
U.S. Pat. No. 6,242,893 issued to Freedman discloses a battery charging circuit which interrupts the charging or discharging of a battery to prevent over-charging or over-discharging.
U.S. Pat. No. 6,268,713 issued to Thandiwe discloses a method wherein a battery charger controller (circuit) detecting a fault isolates one or more batteries while simultaneously notifying a user or host device.
U.S. Pat. No. 5,898,356 issued to Gascoyne et al. discloses a thermally-activated switch which by-passes a cell with an open circuit.
U.S. Pat. No. 6,172,482 issued to Eguchi discloses a battery protection circuit comprising a thermally-activated fuse intended to prevent over-charging.
U.S. Pat. No. 5,684,663 issued to Mitter discloses a resettable circuit intended to protect a battery pack from an external short by disconnecting the battery pack from the faulty load by means of a control FET until the fault is cleared.
Similarly, U.S. Pat. No. 5,625,273 issued to Fehling et al. discloses a latching circuit intended to disconnect an external device from the battery in the event of sensing a fault (over-heating, voltage reversal or short circuit).
Each of the foregoing approaches fails to mitigate over-heating caused by a short circuit internal to a battery (as opposed to battery heating caused by an external short). Such faults are generally addressed in the industry by a so-called “safety separator” made of a porous material that, when heated to a specific temperature, fuses (becomes impermeable) and electrically isolates a cathode from an anode, shutting down the electrochemical reaction. For example, U.S. Pat. No. 6,127,438 issued to Hasegawa, et al. discloses a safety separator made of polyethylene microporous film with high tensile strength, a 20-80%, porosity, a gel fraction of 1% or more and an average pore diameter determined by the permeation method of 0.001-0.1 μm, and a method for producing same. The separator so disclosed fuses (becomes impermeable) at between 100° C. (212° F.) and 160° C. (320° F.). Additionally, the separator disclosed is claimed to have a breaking time of 20 seconds in silicone oil at 160° C. (320° F.). There are numerous other variations of safety separators used in the industry. A short circuit resulting from faulty manufacture, however, such as a contaminant lodged in the components of a battery during assembly creating a hole in the separator, growth of dendrites within the battery or crushing or penetration of the battery can defeat the safety features of the safety separator, causing a runaway condition and overheating.
The large flow of current through such an internal short can cause heat and pressure to rise dramatically inside the battery. Each of the foregoing prior art approaches (both the use of safety circuits and safety separators) further suffers from the problem that the energy stored in the battery may continue to over-heat the battery, causing a build-up in pressure, explosion or combustion resulting in rupture of the battery enclosure, and/or leaks of electrolyte. Thus, a means of preventing a runaway condition in the event of the failure or breach of a safety separator would be highly beneficial.
U.S. Pat. No. 5,478,667 issued to Shackle, et al. discloses a heat dissipation scheme in which the current collectors of a battery serve as a heat sink to help dissipate to the atmosphere heat generated inside the battery. However, such an arrangement is impractical for small batteries, especially those that are medically implanted. Moreover, such a thermal sink would likely not dissipate heat quickly enough in the event of an internal short circuit, especially one caused by a sudden penetration or crushing event. Thus, a passive heat sink such as described in the '667 patent would likely not prevent a runaway condition and may not be adequate to prevent electrolyte leaks, explosions or even combustion.
A better approach not found in the prior art is to provide an emergency energy drain method or device intended to intentionally rapidly deplete stored energy to minimize further battery heating and resulting damage.